This invention relates to a non-crosslinked foam of a composition comprising a major proportion of a 1-butene polymer and a minor proportion of a propylene polymer. More specifically, it relates to a non-crosslinked foam of a composition comprising a 1-butene polymer and a propylene polymer having excellent heat resistance, sealability and heat creep resistance. It is contemplated that this foam could be used as a cap liner or for other applications, such as foam cushion packaging, other flexible packaging end uses, backing for wear surfaces, and other typical foam applications.
In recent years, cans for holding beverages have been superseded by bottles. Ampoules for holding pharmaceuticals are also desired to be replaced by bottles with screw caps because glass fragments are likely to get into the contents upon opening. The most important property required of caps used in such bottles is sealability, and to secure this property, foams of high-pressure low-density polyethylene have gained widespread use as cap liners of thermoplastic resins.
Containers holding pharmaceuticals and drinks are usually subjected to sterilization treatment at high temperatures, or retorting, after filling. The sterilizing temperature has recently tended to become higher for shortening the sterilizing time and increasing productivity, and nowadays, the sterilization treatment is carried out mainly at temperatures as high as at least 100.degree. C.
High-pressure low-density polyethylene foams have low heat-resistant temperatures and cannot be sterilized at high temperatures of at least 100.degree. C. and therefore cannot be used for such an application. On the other hand, polypropylene has excellent heat resistance but a uniform foam of polypropylene alone is very difficult to obtain.
Japanese Patent Publication No. 40,167/1981 discloses a foam prepared from a mixture of 90 to 30% by weight of polypropylene and 10 to 70% by weight of polybutene-1. The patent publication states that this foam has excellent heat resistance, high-temperature creep characteristics, mechanical strength and thermal insulating property and is suitable for use as a thermally insulating material for high-temperature structures. It also states that when the mixing proportions of polyproyplene and polybutene-1 fall outside the above-specified ranges, a good foam cannot be obtained.
Japanese Laid-Open Patent Publication No. 129,025/1983 discloses a polyolefin foam prepared from a mixture of (a) 60 to 98% by weight of a propylene/ethylene block copolymer having a melt index of 0.5 to 2.0 and an ethylene content of 5 to 15%, (b) 1 to 20% by weight of polybutene or a butene copolymer having a melt index of 2 to 10, and (c) 1 to 20% by weight of low-density polyethylene having a melt index of 2 to 10.
It is an object of this invention to provide a novel non-crosslinked foam of a composition comprising a 1-butene polymer and a propylene polymer. The term "non-crosslinked" has the same meaning as "non-vulcanized" which is the foam is produced without vulcanizing (crosslinking) agents or irradiations for vulcanizing (crosslinking). The term "foam" means foamed and expanded materials or sponge-like materials which have many small cells.
Another object of this invention is to provide a non-crosslinked foam suitable for use as a cap liner, such as a bottle closure, having excellent flexibility, sealability, heat resistance, heat creep resistance and hygienic property.
Further objects of this invention along with its advantages will become apparent from the following description.